North Korean leader Kim Jong Un applauds as he visits an athletics contest held by Kim Il Sung University of Politics and Kim Il Sung Military University at a stadium in North Korea.

PYONGYANG, North Korea — North Korea lashed out anew Tuesday at South Korea over a small public protest in Seoul in which demonstrators burned effigies of the North’s leaders, saying it would not hold talks with its southern neighbor unless it apologized for anti-North Korean actions “big and small” and warning that it could take retaliatory measures at any time.

The statement, which was issued by the Supreme Command of the Korean People’s Army, came amid international fears that the North is preparing to conduct a medium-range missile test and also as North Korea marked the second day of festivities in honor of the April 15 birthday of its first leader, Kim Il Sung.

Later in the day, its state media quoted a Foreign Ministry spokesman saying North Korea has no intention of holding talks with the U.S. unless it also abandons its hostility against the North.

The spokesman said the North will “intensify unspecified military countermeasures” unless the U.S. stops conducting military drills on the peninsula and pulls out all the military assets needed to threaten the North with a nuclear attack.

The renewed vitriol, which included the threat for unspecified retaliatory action, followed a Monday protest by about 250 people in downtown Seoul, where effigies of Kim Il Sung and his late son and successor, Kim Jong Il, were burned. Such protests are fairly common in South Korea, and though Monday’s was held on the holiday that North Korea calls “The Day of the Sun,” some analysts suggested North Korea was using it as a pretext to reject calls for a dialogue with the South, at least for the time being.

North Korea often denounces protests like the one held Monday, but rarely in the name of the Supreme Command, which is headed by Kim Il Sung’s grandson and North Korea’s overall leader, Kim Jong Un.

The North’s statement said it would refuse any offers of talks with the South until it apologized for the “monstrous criminal act.”

“If the puppet authorities truly want dialogue and negotiations, they should apologize for all anti-DPRK hostile acts, big and small, and show the compatriots their will to stop all these acts in practice,” the statement said. North Korea’s official name is the Democratic People’s Republic of Korea, or DPRK.

This year’s festivities were mostly low key, with Pyongyang residents gathering in performance halls and plazas and taking advantage of subsidized treats, like shaved ice and peanuts. Last year’s anniversary — the centennial of Kim Il Sung’s birth — was marked with days of immense festivities and a massive military parade.

Instead of such grandiose events, the front page of the Rodong Sinmun, the Workers’ Party newspaper, Tuesday featured photos of Kim Jong Un at an orchestral performance with his aunt, Kim Kyong Hui, and other top officials. North Korean media also reported that he watched volleyball and basketball games between Kim Il Sung University of Politics and Kim Il Sung Military University.

But word of the protest in the South outraged some North Koreans, though North Koreans, too, have similarly used the image of South Korean officials in protests against Seoul. The former South Korean president was depicted as a rat, shown attacked by dogs and tied up and quartered. The North Korean military used his image for military practice

“A child will not ignore it if his parents are insulted,” said Pyongyang resident Ri Jong Chol. “I’d like to say that we have to find the South Korean puppet traitors who insulted our top leaders, wherever they are, and put them to death.”

After Pyongyang’s latest volley of rhetoric, South Korean Defense Ministry spokesman Kim Min-seok said South Korea was closely monitoring its moves and would “thoroughly and resolutely punish North Korea if it launches any provocation for whatever reason.”

The calm over the past two days in Pyongyang has been a striking contrast to the steady flow of retaliatory threats North Korea has issued over ongoing military exercises between South Korea and the United States. Though the maneuvers, called Foal Eagle, are held regularly, North Korea was particularly angry over their inclusion this year of nuclear-capable B-2 stealth bombers and F-22 fighters.

“The ultimatum is just North Korea’s way of saying that it’s not willing or ready to talk with the South,” said Chang Yong-seok at the Institute for Peace and Unification Studies at Seoul National University. “North Korea apparently wants to keep the cross-border relations tense for some time to come.”

The Tuesday ultimatum comes just after U.S. Secretary of State John Kerry wrapped up a tour to coordinate Washington’s response with Beijing, North Korea’s most important ally, as well as with Seoul and Tokyo. Kerry said a missile test would be provocation that would further isolate the country and its impoverished people. He said Sunday that the U.S. was “prepared to reach out,” but that Pyongyang must first bring down tensions and honor previous agreements.

South Korean Defense Minister Kim Kwan-jin told a parliamentary committee Monday that North Korea still appeared poised to launch a missile from its east coast. North Korea, which conducted a nuclear test in February, has already been slapped with strengthened U.N. sanctions for violating Security Council resolutions barring the regime from nuclear and missile activity.

To further coordinate their response, South Korea’s new president, Park Geun-hye, will meet with President Barack Obama on May 7 at the White House.

“While we take these provocations from North Korea seriously, the Park Geun-hye government remains calm yet resolute vis-a-vis North Korea,” Foreign Minister Yun Byung-se told reporters in Seoul on Monday. “Based on the overwhelming combined deterrent capability of the Korea-US alliance and solidarity of the international community, our frontline is secure, and our society remains calm and stable.”

The U.S.-South Korean military drills are scheduled to end April 30. On Tuesday, a Marine CH-53E helicopter made a “hard landing” during the exercises, according to a statement from United States Forces Korea. Twenty-one personnel were on board the helicopter, including five crew members, the statement said. All were taken to the hospital, but 15 were quickly released. The remaining six were in stable condition.